Radio frequencies at the very low end of the electromagnetic spectrum can be useful for a number of defense applications. Ultra-Low Frequencies (ULF), e.g., from 100 Hz to 3 kHz, are particularly useful for penetration of signals through conductive media such as water, metal, soil, rock, and building materials. Penetration is possible due to the relatively large skin depth in these materials which grows as the carrier frequency is reduced. The Very Low Frequency (VLF), e.g., from 3 kHz to 30 kHz, band is likewise of interest for defense communications because signals in this frequency range couple to the ionosphere-ground waveguide surrounding the Earth. The waveguide can facilitate the long range propagation of signals around the globe with very little attenuation. However, the free-space wavelengths of electromagnetic fields at ULF and VLF frequencies measure tens to thousands of kilometers in length, resulting in either very large or severely inefficient transmitter structures when constructed using conventional antenna approaches. Such transmitters may be impractical in many operational scenarios, especially those requiring physical mobility.
While the present invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description of exemplary embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the embodiments above and the claims below. Reference should therefore be made to the embodiments above and claims below for interpreting the scope of the invention.